Applications involving the transmission of radio frequency (RF) energy (such as, for example, microwave or millimeter wave energy) through free space are abundant. For example, radar systems, satellite communications systems, aircraft altimeter and guidance systems, and ground reconnaissance mapping systems all involve the transmission of RF energy through space. To implement such systems, antennas must be provided for radiating and/or receiving the RF energy to/from free space. In this regard, the antenna acts as a transition between a wave guiding structure (i.e., a transmission line) internal to the system and free space. Many different types of antennas exist, each having its own advantages and disadvantages.
In many systems, both commercial and military, multiple applications involving the transmission of RF energy are practiced. For example, commercial aircraft generally include both weather radar units and ground communications systems. In such systems, at least one antenna is required to perform each application. A problem arises when limited surface space (i.e, real estate) is available for the antennas, such as is generally the case with aircraft. In general, it is difficult to implement multiple antennas in close proximity to one another because of interference and crosstalk concerns.
Therefore, a need exists for a method and apparatus for implementing multiple antennas within a limited space without incurring negative interference effects. Also, a need exists for a method and apparatus for increasing the number of antennas that may be implemented within a given space.